1) A singular subject requires a singular verb. A plural subject requires a plural verb with a few exceptions.
I sing. You sing. We all sing for ice cream.
The little girls all sang for their supper.
2) If the subject has two singular nouns joined with and use a plural verb.
Dick and Jane are ready to go home.
3) If the subject has two singular nouns joined with or or nor, use a singular verb.
Neither Dick nor Jane is ready to go home.
4) If the subject has a singular noun joined to a plural noun by or or nor, the verb should agree with whichever noun comes last.
Neither Dick nor his friends want to play catch outside.
Either Sally or Jane visits everyday.
5) The contractions doesn't (does not) and wasn't (was not) are always used with a singular subject.
Dick doesn’t want to go.
6) The contractions don't (do not) and weren't (were not) are always used with a plural subject. The exception to this rule is I and you require don't.
We don’t want to go with Jane.
You don’t believe me.
I don’t want to go home yet.
Dick, as well as his friends, hopes the Colts win.
Jane, as well as Sally and Dick, hopes the meeting will be over soon.
Each of them will go there someday.
Nobody knows Dick is here.
Either way works.
Neither option is viable.
9) Plural nouns functioning as a single unit, such as mathematics, measles, and mumps, require singular verbs. An exception is the word dollars. When used to reference an amount of money, dollars requires a singular verb; but when referring to the bills themselves, a plural verb is required.
Five thousand dollars would suffice.
Dollars are easier to exchange than Euros.
Dick's trousers are worn.
Jane's scissors are missing.
There are many who would agree with you.
There is the question of who goes first.
Dick, accompanied by his wife Jane, will arrive in ten minutes.
Everything, including the kitchen sink, is up for auction.
The cousins, together with their dog, are going to be here for a week.